The present invention relates to vacuum interrupter electrical apparatus and more importantly to the electrical contacts of such apparatus. Vacuum interrupters find application as circuit protection devices in electrical transmission and distribution systems. The device basically comprises movable contacts disposed within an evacuated sealed envelope, with the contacts being closed in a current carrying configuration and separated or opened to interrupt the operational current.
The selection of materials for the electrical contacts of a vacuum interrupter significantly determine the operational characteristics of the vacuum interrupter. The contacts are typically made of composite materials frequently made by mixing a powder of a high conductivity material with a powder of a high melting point material, pressing them together, and sintering the compact to form a strong contact. The combination of such powder materials results in the contact having good conductivity characteristic for normal carrying operation, and low erosion characteristic when the contacts are separated in the vacuum and an interrupting arc is extinguished to interrupt the operating current. It is well known that the breakdown voltage characteristic of such contacts when separated in vacuum is related to the composition materials of the contact. By breakdown voltage is meant the voltage at which a restrike of an arc is formed between the separated contacts due to the voltage which appears across the separated contacts. It is well known that high conductivity contact materials such as copper and silver have a much lower breakdown voltage characteristic than the higher melting point materials such a chromium and tungsten. A widely used contact material is a blend of a high conductivity material such as copper and a high melting point refractory metal such as chromium or tungsten. A widely used chromium matrix contact material which is infiltrated with copper is described in U.S. Pat. No. 3,818,163. It is well known that contacts made from copper and chromium mixtures display breakdown voltage characteristic which is somewhere between that for the copper or chromium alone.
In more recent copper chromium vacuum contacts a higher percentage of copper, in the range of 65 to 90 wt.% of the contact, has been fabricated as disclosed in U.S. Pat. No. 4,190,753. This high copper content is desirable from an operating current carrying efficiency point of view, and to provide improved capability for interrupting fault current, but has been found to lower the voltage breakdown characteristic of the contact.
It is therefore desirable to be able to fabricate a high copper content vacuum interrupter contact material which exhibits improved voltage breakdown characteristic while retaining the high conductivity characteristic of such contacts.
The dispersion strengthening of metals including copper is known in the art as described in "Modern Developments in Powder Metallurgy", edited by H. H. Hausner, Vol. 5 Materials and Properties, Plenum Press, 1971, which includes a chapter titled Dispersion Strengthening.